The present invention relates to an optical information recording and reproducing apparatus to record/reproduce information in/from an optical information recording medium by using a holography.
In these days, with the Blu-ray Disc (BD) standard and the High Definition Digital Versatile Disc (HD DVD) standard, optical discs having as large as 50 GB recording density can be produced on a commercial basis in a consumer use by using a blue-violet semiconductor laser.
In the future, it is desired that the optical disc has a large recording capacity such as HDD (Hard Disc Drive) having as large as 100 GB to 1 TB.
However, for a purpose of realizing the above-mentioned ultra-high density optical disc, a new storage technique is required such that it is different from an old trend using an existing high density technique that depends on making a wavelength shorter and the NA of an object lens higher.
On researching into a next-generation storage technique, a hologram recording technique has been taken notice of recording digital information by using the holography.
The hologram recording technique is that a signal light having information of page data modulated two-dimensionally by a spatial light modulator is superposed with a reference light inside a recording medium, and an interference fringe pattern occurs at this time to thereby arise a refraction index modulation inside the recording medium to thereby record information.
On reproducing information, the reference light used for recording is irradiated onto the recording medium with the same setup of the reference light. Consequently, a hologram recorded in the recording medium acts as a diffraction grating to generate a diffracted light. This diffracted light is reproduced as an identical light containing the recorded signal light and phase information.
A reproduced signal light is detected two-dimensionally at a high speed by using an optical detector such as CMOS, CCD, etc. The above-mentioned hologram recording is effective for recording and reproducing a large amount of information at a high speed, since two-dimensional information is recorded and reproduced simultaneously by using a single hologram and a plural number of page data can be overwritten on the same position.
JP-A-2004-272268 (US2006/238841) has proposed a hologram recording technique. This document discloses a so-called angle multiplexing recording system in which a signal beam is converged onto an optical information recording medium by a lens and, at the same time, the reference light of a parallel beam is irradiated thereon so as to be made interfered with the signal beam to record a hologram, and further, in order to execute a multiple recording, different page data is displayed on the special light modulator, while an incident angle of the reference light toward the optical recording medium is varied. JP-A-2004-272268 also discloses a technique in which the signal light is converged on the optical information recording medium by a lens to arrange an opening or aperture (spatial filter) on its light waist position, so that an interval adjacent to the hologram can be made short, and the recording density or capacity can be made increased, compared with the existing angle multiplexing recording system.
WO2004-102542 (US2008/192311) has also proposed a hologram recording technique. This document discloses a technique using a shift multiplex system in which a light coming from an inner side of pixels is set to the signal beam in a single spatial light modulator and a light coming from an outer side of orbicular zone pixels is set to the reference light therein. Both of the light beams are converged onto an optical recording medium by using the same lens so as to make the signal and reference lights interfered at a vicinity of a lens focal plane to thereby record a hologram.